1. Field of the Invention
The present invention relates to a light-emitting device using an organic compound. More particularly, the present invention relates to an organic electroluminescent device (organic EL device).
2. Related Background Art
Applied research has been vigorously made on an organic EL device as a high-responsiveness and high-efficiency light-emitting device (Macromol. Symp. 125, 1 to 48 (1997)). FIGS. 1A and 1B each show the basic structure of the device. As shown in FIGS. 1A and 1B, the organic EL device is generally structured such that an organic multi-layer is sandwiched between a transparent electrode 5 on a transparent substrate 6 and a metal electrode 1.
In FIG. 1A, the organic layer is composed of an electron-transporting layer 2, a light-emitting layer 3, and a hole-transporting layer 4.
For example, ITO having a large work function is used for the transparent electrode 5 to provide good property of injecting a hole from the transparent electrode 5 to the hole-transporting layer 4. A metal material having a small work function such as aluminum, magnesium, or an alloy using any one of them is used for the metal electrode 1 to provide good property of injecting electrons to the organic layer. Those electrodes each have a thickness in the range of 50 to 200 nm.
For example, an aluminum-quinolinol complex (typified by Alq3 shown below) having electron-transporting property and light-emitting property is used for the light-emitting layer 3. In addition, a material having electron-donating property such as a biphenyl diamine derivative (typified by α-NPD shown below) is used for the hole-transporting layer 4. An oxadiazole derivative or the like can be used for the electron-transporting layer 2.
Fluorescence upon transition of a singlet excitaton of a molecule as a light-emitting center to a ground state has been heretofore taken as light emission generally used in an organic EL device. Meanwhile, a device utilizing not fluorescent emission via a singlet excitaton but phosphorescence via a triplet excitaton has been under investigation (“Improved energy transfer in electrophosphorescent device” (D. F. O'Brien et al., Applied Physics Letters Vol 74, No 3, p 422 (1999) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” (M. A. Baldo et al., Applied Physics Letters Vol 75, No 1, p 4 (1999)). In each of those documents, a four-layered structure composed of organic layers shown in FIG. 1B is mainly used. The four-layered structure is composed of a hole-transporting layer 4, a light-emitting layer 3, an excitaton diffusion preventing layer 7, and an electron-transporting layer 2 from the side of an anode. The materials used are the following carrier-transporting materials and phosphorescent materials. Abbreviations of the respective materials are as follows.
Alq3: aluminum-quinolinol complex
α-NPD: N4,N4′-Di-naphthalen-1-yl-N4,N4′-diphenyl-biphenyl-4,4′-diamine
CBP: 4,4′-N,N′-dicarbazole-biphenyl
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
PtOEP: platinum-octaethylporphyrin complex
Ir(ppy)3: iridium-phenylpyridine complex

High efficiency was obtained in each of “Improved energy transfer in electrophosphorescent device” (D. F. O'Brien et al, Applied Physics Letters Vol 74, No 3, p 422 (1999) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” (M. A. Baldo et al, Applied Physics Letters Vol 75, No 1, p 4 (1999) because α-NPD was used for the hole-transporting layer 4, Alq3 was used for the electron-transporting layer 2, BCP was used for the excitaton diffusion preventing layer 7, and CBP as a host material mixed with PtOEP or Ir(ppy)3 as a phosphorescent material at a concentration of about 6% was used for the light-emitting layer 3.
A phosphorescent material has been attracting considerable attention because it is expected to provide high luminous efficiency on principle. The reason for this is that excitatons generated by carrier recombination are composed of singlet excitatons and triplet excitatons, and the ratio between the number of singlet excitatons and the number of triplet excitatons is 1:3. An organic EL device utilizing a singlet has taken fluorescence upon transition from a singlet excitaton to a ground state as light emission. However, on principle, the luminescence yield of the device was 25% of the number of generated excitatons, and the value was an upper limit on principle. When phosphorescence from an excitaton generated from a triplet is used, an yield at least 3 times as high as that of the above yield is expected on principle. Furthermore, when transfer due to intersystem crossing from a singlet at a higher energy level to a triplet at a lower energy level is taken into consideration, a luminescence yield 4 times as high as the above yield, that is, a luminescence yield of 100% is expected.
In addition, the development of a host material using a phosphorescent metal coordination compound as a dopant has been actively made (Japanese Patent Application Laid-Open No. 2003-55275). However, when Ir(ppy)3 described in it is used as a dopant, a threshold voltage is high and a current is hard to flow, so the host material is susceptible to improvement.
In the above-described organic EL device using phosphorescence, injection of an increased number of carriers to the light-emitting layer at a low voltage while maintaining a balance between an electron and a hole at a low voltage is important for the achievement of high luminance and high efficiency. However, some of the above phosphorescent materials have low charge-injecting/transporting properties and are hard to allow a large amount of current to flow at a low voltage.